1. Field of the Invention
This invention relates to a method for producing acrylic acid comprising introducing propylene as a raw material in high concentration into a reactor which has a first reaction zone and a second reaction zone formed by dividing with at least one perforated tube plate, and improving water concentration in an acrylic acid-containing solution in an acrylic acid absorption column to a level in the range of 1–45 wt. %, thereby preventing the solution from polymerization at the subsequent steps in the process of the production.
2. Description of the Related Art
Acrylic acid is used in coating material, fiber processing, leather processing, and building material besides being used for acrylic fiber copolymer or as an emulsion for adhesive agents. The demand for this compound is now increasing. Thus, it is generally produced by the reaction of catalytic gas phase oxidation of propylene, for example, with a view to realizing the principle of mass production using an inexpensive raw material. The raw material gas is generally supplied at a concentration in the range of about 4–7 vol. % because the reaction of catalytic gas phase oxidation has the possibility of belonging to the range of explosion, depending on the proportion of molecular oxygen to be used in the reaction of oxidation. Since the raw material gas is preferred to be used at a high concentration for the purpose of improving the efficiency of production, various modifications have been proposed for the supply of the raw material gas at a high concentration.
The official gazette of JP-A-2000-103761, for example, discloses a method for producing acrylic acid from propylene by supplying a reaction composition comprising more than 7 vol. % of propylene, molecular oxygen, steam, and the balance of inert gas to a reactor furnished with two reaction zones packed with a catalyst and having a multiplicity of reaction tubes duly distributed therein. When the mixed product gas obtained by the reaction of catalytic gas phase oxidation is introduced into an absorber to obtain an aqueous acrylic acid solution, the exhaust gas of the absorber contains unreacted propylene, unreacted acrolein, and acrylic acid as residues. When this exhaust gas is put to cyclic use, the coefficient of absorption is depressed and the yield of acrylic acid is degraded because the volume of the inert gas flowing through the absorber is increased thereby being generated a load in the top of the absorber. Conversely, when the exhaust gas is not put to cyclic use, the water content in the gas flowing through the absorber is varied because the selectivity, the ratio of addition, and the catalytic activity in the reaction of oxidation are affected by the water content owing to the use of steam as a diluting gas. The method disclosed in the official gazette mentioned above contemplates reducing the load exerted on the absorber by using propylene of higher concentration than usual heretofore in the system of a single reactor and decreasing the exhaust gas of the absorber held as necessary for the sake of dilution.
Incidentally, after the absorption of the acrylic acid-containing gas, such purification steps as the dehydration of the acrylic acid-containing solution and the separation of low boiling substance and high boiling substance contained therein are carried out.
Acrylic acid, however, is an easily polymerizing compound. When the raw material gas of high concentration is used with a view to heightening the productivity of the method, this method is liable to generate an acrylic acid polymer at the step for absorbing acrylic acid and the subsequent step for purification of the absorbed acrylic acid. Since various columns for purificating continue production of acrylic acid while preventing occurrence of polymerization by adjusting such factors as the pressure of distillation, temperature, and amount of feed, the control of these factors is not easy because changes in these conditions affect other conditions. When the concentration of acrylic acid is depressed by a variation in the composition of the bottom liquid of an absorption column, for example, the maintenance of distillation conditions in a high acrylic acid concentration entails generation of an acrylic acid polymer in a distilling column. Though it is not to the extent of requiring forced suspension of the operation due to blockage, but possibly results in degrading the quality of acrylic acid produced due to the inclusion of the polymer and consequently depressing the yield of the product.
The purification step of acrylic acid is accomplished more often than not by adopting distilling columns called a dehydrating column, a light ends cut column, and a heavy ends cut column, concatenating the plurality of distilling columns endowed with different functions, and operating them jointly thereby performing a series of purification. Thus, the control of this purification step is rendered further difficult by the fact that a variation in the composition of the bottom liquid of any one of such distilling columns necessitates a due change in the purification conditions of the subsequent steps.
The solution, the exhaust gas, and the like which are discharged from the step for producing acrylic acid at times contain the raw material compound, the product, and other useful compounds. The cyclic use of such discharged substances in the process of production, therefore, can improve the yield of production. The composition of the exhaust gas, for example, is varied as with the distillation conditions and the generation of the polymer proceeds like a chain reaction. In the method for the production of acrylic acid particularly included a step for recycling the exhaust gas, therefore, it is extremely difficult to control the distillation conditions constant.